Dude don't even mess with heat right away. First 10-14 days you def want to apply ice. Then you can start rotating in heat. 20 minutes ice, 20 minutes nothing, 20 minutes heat, 20 minutes nothing. Repeat.
Take it easy and go by feel, You don't want to aggrivate it and make it worse. im a master at aggrivating minor injuries, its just not worth it to keep lifting. Take a week off and if it has felt fine for a few days, start up again, if not, another week off. im dealing with a strained lower back right now and havent ben able to lift for 2 months!!! Im also having to go thru physical therapy. It aint fun, and yeah, it started as a pulled muscle.